The present invention relates to aligning an optical fiber to a light source and more particularly, to aligning a polarization maintaining optical fiber to the polarization of a light source.
In performing various measurements on polarization maintaining optical fibers and devices that use polarization-maintaining fibers it is desirable to align the polarization of the light launched into the fiber to the fiber""s polarization axis. For example in the measurement of a fiber Bragg grating, the spectral center wavelength of the grating""s reflection band is dependent on which polarization is launched into the fiber. Furthermore, if a conventional spectrum analyzer is used, the polarization dependence of the spectrum analyzer response may cause measurement errors. Therefore one needs to launch light into the fiber so that the polarization is aligned to the fiber axis. Fiber loop polarization controllers are often used to adjust the polarization of the light going into the grating. A conventional measurement method, as described above, requires as much as 15 minutes per fiber.
Other methods of aligning polarization axes of fibers exist, but may be unsuitable in certain applications. In particular, if the light source is a tunable laser, some of the other conventional methods will not work, due to the laser""s high temporal coherence. For example, in a conventional approach, described by Miles (U.S. Pat. No. 4,673,244), a highly coherent light source can give false readings. In this approach, light is launched into the fiber and the polarization state from the fiber output is measured. The process is repeated until the output light is highly polarized, indicating that the fiber polarization axis is aligned. This method may not always work. The Miles patent states, xe2x80x9cThe birefringence of the fiber creates a remote possibility that laser light received into the fiber between the axes of propagation will emerge from the fiber as linearly polarized light.xe2x80x9d Furthermore, this method may be very time-consuming.
In another conventional method (for example Anjan, et al, U.S. Pat. No. 5,245,400), polarized light is launched into the fiber aligned to the fiber""s axis and the orientation of the output end of the fiber is determined by squeezing the fiber using a mechanical transducer. While this method can locate the fiber axis at any position along the length of the fiber, it requires a priori alignment of the light source to the fiber and therefore is unsuitable for aligning the light source itself. Also, the method relies on the ability to rotate the fiber. Therefore it is not well suited for the case whether it is desirable to hold the fiber fixed and rotate the polarization of the light source.
In another conventional method, light is directed perpendicularly through the side of the fiber and the light passing through the fiber is analyzed with a computer to determine the orientation of the fiber axes. (U.S. Pat. Nos. 5,417,733; 5,488,683; 5,625,735; 5,758,000). This method is practical in fusion splicing polarization-maintaining fibers. However, the method relies on the ability to rotate the fiber. Therefore it is not well suited for the case where it is desirable to hold the fiber fixed and rotate the polarization of the light source, such as is the case when a fiber ribbon is used.
The invention is a substantial improvement over conventional methods of aligning the polarization. The conventional processes may be difficult, time consuming, inaccurate, and do not always offer proper alignment of the polarization.
Briefly, the invention includes a method and apparatus for aligning plural polarization-maintaining optical fibers with a polarized light source, using the fact that in the case of misaligned elements the output power will vary with wavelength. The subject method and apparatus involves using a polarizing filter at the output of the fibers, rotating the polarization of the light launched into the fibers, and varying the light wavelength, by computer control, until an output spectrum ripple is eliminated, as further described and illustrated.